Fixed issues with definition of handler404 and handler500

Django 1.10 wants urls to have the view they are passed as a
callable. In addition, the view didn't define server_error,
it only defined server_error_500. Switched to using that.

readthedocs/core/urls/subdomain.py

@@ -10,14 +10,16 @@
 
 from readthedocs.core.views.serve import (
     redirect_page_with_filename,
-    redirect_project_slug,
-    serve_docs
+    redirect_project_slug, serve_docs
+)
+from readthedocs.core.views import (
+    server_error_500,
+    server_error_404,
 )
-
 from readthedocs.constants import pattern_opts
 
-handler500 = 'readthedocs.core.views.server_error'
-handler404 = 'readthedocs.core.views.server_error_404'
+handler500 = server_error_500
+handler404 = server_error_404
 
 subdomain_urls = [
     url(r'^(?:|projects/(?P<subproject_slug>{project_slug})/)'
@@ -33,8 +35,8 @@
          r'(?P<lang_slug>{lang_slug})/'
          r'(?P<version_slug>{version_slug})/'
          r'(?P<filename>{filename_slug})$'.format(**pattern_opts)),
-         serve_docs,
-         name='docs_detail'),
+        serve_docs,
+        name='docs_detail'),
 ]
 
 groups = [subdomain_urls]